As a transparent conductive film, a film comprising a plastic film and a conductive material provided thereon is commonly used. As the conductive material, either organic or inorganic materials can be used. Inorganic materials are preferable in terms of both conductivity and transparency. As the inorganic material, metals, such as gold, silver and the like, and metal oxides are preferable in terms of transparency. Among metal oxides, indium oxide, tin oxide, zinc oxide, and mixed oxides thereof are particularly preferable. Films in which the above-described metal oxide is deposited on a plastic film using a vapor deposition method, an ion plating method, a sputtering method or a CVD method, are known.
Transparent conductive films are generally produced by an ion plating device or a sputtering device which roll up a film. A transparent conductive film roll produced by the above-described device is cut by a slitter into pieces having a size of about 300 to 800 mm in width and about 10 to 1000 m in length, which are in turn rolled up by a paper tube or plastic core. Thus, the transparent conductive film is generally circulated in the form of a film roll. After a film roll in which a film is rolled up is cut into sheets, silver paste printing, dielectric printing or the like is performed on the film so that the resultant film is used as a transparent electrode for a tough panel or an electroluminescence panel.
In analog touch panels, the position of an input is recognized and characters or symbols are displayed, assuming that the distribution of surface resistance of a transparent electrode thereof is uniform (“Gekkan Disupurei [Monthly Display”, September 1999, p. 82). Therefore, the surface resistance of a transparent conductive film used therein needs to be uniformly distributed at any position thereof. Also, in the case of transparent electrodes of electroluminescence panels, a transparent electrode having a uniform surface resistance distribution is required to obtain uniform light emission intensity within the panel. In particular, as the size of an electroluminescence panel is increased, the higher degree of uniformity is required for the distribution of surface resistance of a transparent electrode thereof.
The distribution of surface resistance of a transparent electrode can be made uniform as follows. A surface resistance measuring device is provided in a rolled-up film forming device. The surface resistance of a transparent conductive layer is sequentially measured in-line while forming the transparent conductive layer. Conditions for forming the transparent conductive layer are regulated so that the surface resistance thereof is uniformly distributed.
An example of the above-described method includes a method for contacting and sandwiching a transparent conductive film between two metal rollers and measuring the surface resistance between the rollers. However, in principle, this method can measure the distribution of surface resistance of a transparent conductive film in a longitudinal direction thereof, but not the surface resistance distribution in a lateral direction thereof. Concerning the surface resistance distribution in a longitudinal direction thereof, if the tension of the film is not uniform, the contact between the metal rolls and the transparent conductive layer is not uniform, leading to errors in the measurement of the surface resistance.
There is also a method for measuring the surface resistance of a transparent conductive film in a lateral direction thereof, in which three or more metal rings are provided around an insulated roller (made of silicone rubber or polytetrafluoroethylene) and the resistance between each metal ring is measured. However, a small protrusion is formed between the insulator and the metal ring, which is likely to damage the film surface.
Therefore, as a surface resistance measuring device which can sequentially measure the surface resistance distribution in a lateral direction thereof and does not damage the film surface, a method for measuring a coupled inductance between an electromagnetic induction coil and a conductive film (a method for applying magnetic field and measuring a resulting eddy current) is known (“Gekkan Disupurei [Monthly Display”, September 1999, p. 88). In this method, however, a considerably high intensity of applied magnetic field is required for the measurement of a conductive film having a surface resistance of 10Ω/□ or more. In this case, the spread of magnetic flux is large, leading to the path line fluctuation of a substrate in a production process (vibration in a direction normal to a surface of the substrate). Thus, a distance between a sensor section and a conductive film to be measured fluctuates and the coupled inductance is not constant. As a result, the in-line sequential measurement has large measurement errors.
Further, in this method, the magnetic permeability of a ferrite coil which functions as an eddy current generating section or an eddy current detecting section has a temperature dependency. The inductance is changed in accordance with, if any, the fluctuation of the temperature. Therefore, even if a high frequency voltage applied to the coil is constant, an eddy current flowing through the conductive film is changed, resulting in large measurement errors.
As described above, even if a general surface resistance measuring device is provided in a rolling up device, large measurement errors make it considerably difficult to obtain a transparent conductive film roll having uniform surface resistance.
The present invention was made, taking the above-described circumstances into consideration. An object of the present invention is to provide a transparent conductive film roll having a uniform surface resistance distribution in a longitudinal direction thereof and in a lateral direction thereof; a production method thereof; and a touch panel produced using the same.